docs: def __undo_filter_average(self, scanline):
          """Undo average filter."""
          ai = -self.fu
          previous = self.prev
          for i in range(len(scanline)):
              x = scanline[i]
              if ai < 0:
                  a = 0
              else:
                  a = scanline[ai]  # result
              b = previous[i]
              scanline[i] = (x + ((a + b) >> 1)) & 0xff  # result
              ai += 1